Rhabdoid tumor of kidney (RTK) is a rare, highly malignant childhood neoplasm of uncertain histogenesis. Several recent studies have described considerable histochemical heterogeneity among cases of RTK, with confusing combinations of epithelial, mesenchymal, myogenous, and neuroepithelial markers in some tumors. The present study characterizes the histology, ultrastructure, histochemistry, cytogenetics, and oncogene expression in a cell line derived from RTK. The surgical specimen, nude mouse xenograft, and cell cultures demonstrated characteristic intermediate filament whorls by electron microscopy and expressed vimentin (diffusely) and cytokeratin (focally, in hyaline cytoplasmic inclusions) without detectable desmin, Thy-1, or epithelial membrane antigen. S-100 protein was absent in the surgical specimen and heterotransplant, and was seen very weakly and focally in the cell cultures. Light microscopic features of cultures were unchanged by several compounds (tissue plasminogen activator, nerve growth factor, cyclic adenosine monophosphate) which induce differentiation of some other pediatric neoplasms. The growth factor requirements of RTK cultures indicate a cell with mesenchymal features. Insulin-like growth factor-2 mRNA was detected in the RTK and in three Wilms' tumors also studied. Unlike most Wilms' tumors, RTK expresses the c-myc rather than the N-myc oncogene.